Acid-Base and Electrolyte Instrumentation Flashcards
General composition of indicator electrodes
- Ion sensitive membrane
- Electrolyte solution (KCl or HCl)
- Metal wire (AgCl)
- Metal element (silver)
- Cable to meter
Purpose of ion selective electrodes
To generate potential to ion of interest and exclude all others
Measurement principle of typical pH glass membrane electrode
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Two basic functions of reference electrode
- Provide stable potential
- Complete circuit
Components of reference electrode
- Salt bridge → allows ions to enter solution
- Electrolyte solution w/ HIGH concentration
- Metal salt → AgCl wire
- Metal element → Ag
- Cable to meter
Composition of saturated calomel electrodes
Do not use saturated calomel b/c of mercury and asbestos (carcinogens)
Composition of silver/silver chloride electrodes
Ag/AgCl is used today w/ increased concentration of KCl
The measurement of current or amperage created as the voltage applied to an electrochemical cell is changed (e.g., oxygen and serum glucose electrodes)
Voltammetry
MEASURING AMPHERES. That branch of electrochmistry in which a fixed charge is applied to an electrochemical cell. The charge serves as the generating or excitation signal; the response is the current measured. Molecules are oxidized or reduced at the working electrode, whose potential is controlled relative to a reference electrode.
Amperometry
A voltammetric technique in which analysis is based on current/voltage curves produced during electrolysis of a solution. (electrolysis produces chemical changes by passage of an electrical current through an electrolyte)
Polarography
Antibiotic that is used in potassium ion selective electrodes
Valinomycin
Principle of direct ion selective electrodes
Measures undiluted sample
Principle of indirect ion selective electrodes
Measures pre-diluted sample
How does hyperproteinemia and hyperlipidemia interfere with sodium concentrations on both ion selective electrodes?
Interferes w/ both
- Indirect is more affected by both b/c fat makes sample a smaller amount
How can you correct for lipemia when sodium is affected?
Ultracentrifuge
Essential components of a blood gas analyzer
- 3 measuring electrodes (pH, pCO2, pO2)
- Waste pump system
- Calibration component (2 different concentrations of pH, O2, CO2)
- Humidifying system
The number of hydrogen ions needed to raise or lower 1 Liter of whole blood to a pH of 7.40; excess of bicarbonate
Base excess
A whole blood sample is hemolyzed and 4 wavelengths of light pass through the sample
Co-Oximeter
What is measured w/ a Co-Oximeter?
- CO Hgb
- Oxy Hgb
- Met Hgb
- Reduced Hgb/total Hgb is calculated
Properties that depend only on the number of particles in solution, not on the nature of those particles; used in osmolality
Colligative properties
Four colligative properties
- Freezing point
- Vapor pressure
- Boiling point
- Osmotic pressure
Temperature at which the vapor pressure of the solid and liquid phase are the same
Freezing point
Pressure at which the liquid solvent is in equilibrium with the water vapor
Vapor pressure
Temperature at which the vapor pressure of the solvent reaches one atmosphere
Boiling point
Pressure that allows solvent flow between a semi-permeable membrane to establish an equilibrium between compartments of different osmolality
Osmotic pressure
Is freezing point increased or decreased when solute is added to solution?
Decreased
Is vapor pressure increased or decreased when solute is added to solution?
Decreased
Is boiling point increased or decreased when solute is added to solution?
Increased
Is osmotic pressure increased or decreased when solute is added to solution?
Increased
Units for osmolarity
mOsm/L of H2O
Units for osmolality
mOsm/kg of H2O
`Theory of operation of a freezing point depression osmometer
The temp of the sample chamber is slightly lowered until below freezing point at which time the stirring rod is activated to promote crystal formation. When temp stablilizes it is read as the freezing point depression.
Essential components of a freezing point depression osmometer
- Electrode in bath of ethylene glycol (antifreeze)
- Stir wire causes immediate freezing w/ vibration
- Thermistor probe measure heat of fusion release by formation of crystals
- Sample cup
The coulometric titrator consists of the ____ electrode and ____ electrode pair
Indicator; generator
This electrode is made of Ag and is the coulometric part of the titrator
Generator electrode
This electrode is the amperometric part of the titrator; senses end of the rxn of free Ag ions
Indicator electrode
The ____ electrode senses the current and shuts down the titration when the end point is reached
Indicator electrode
this electrode is immersed in a soln containing dissolved CO2 gas; the silicone rubber membrane allows the gas to pass into an internal sodium bicarbonate buffer soln. A pH indicator and reference electrode are immersed in the buffer soln. CO2 reacts with the buffer and the pH change is sensed to measure the CO2.
Stow and Severinghaus (pCO2 electrode)
This electrode is immersed in a solution containing dissolved O2 gas, polypropylene membrane allows O2 to apss into the internal phosphate buffer soln; O2 then reacts with polarized platinum cathode (high sensitivity and fast response); silver anode provides the oxidizing electrode to complete the circuit.
Clark (pO2 electrode)
As O2 is consumed we get a negative charge which is proportional to pCO2 in blood gas; polarographic electrode with oxidation/reduction reactions
Clark (pO2) electrode…shorter description
O2 + 2 H2O + 4e- → 4 OH-
Oxidation reduction rxn in Clark electrode
CO2 + H2O ↔ HCO3 + H3O
Changes in pH in Stow and Severinghaus electrode
More CO2 = ______ in pH and ______ H ions in pCO2 electrode (S&S)
Decreased pH and increased H+
Chemical reactions occuring because of the flow or presence of electrons. It is thus governed by the actual transfer of energy from electrons to chemicals, or the potential for such transfer. The chemicals are in contact with the electrons at surfaces or interfaces.
Electrochemistry
Describes the flow of electrons through a substance, such as a solution or a wire, and is measured in amperes, or coulombs of change per second.
Current
The opposition of a substance to current flow, measured in ohms
Resistance
The ability to do work, the degree of electrification as compared to some standard. for current to flow, the potential applied to the system must be greater than the resistanace exhibited by that system.
Potential
The measurement of the potential difference between two electrodes measured at equilibrium and with no current. the potential generated is proportional to the activity (conc) of the ion being measured.
Potentiometry
A bridge of a salt soln, usually KCl, placed between the two half-cells of a galvanic cell, either to reduce the potential of the liquid junction between the solns of the two half cells to a minimum or to isolate a soln under study from a reference half-cell and prevent chemical precipitations.
Salt bridge
